The three options side by side
There are three common ways to work out what size generator you need: an online calculator, a printed wattage chart, or an app on your phone. They all add up the same two numbers, running watts and starting watts. The difference is where and when you can use them.
On a small screen, scroll the table sideways to read every column.
| Option | Works offline | Starting watts | Best for |
|---|---|---|---|
| Online calculator (like the one on this site) | No, you need a connection to open it | Counted, if the tool asks for starting watts. This site adds the single biggest starting kick. | Planning at home before you buy |
| Printed wattage chart | Yes | You add it by hand | A quick look-up, or a sheet to keep with the generator |
| Generator Wattage Calculator (iPhone) | Yes | Counted automatically | Checking what you can plug in during an outage, at a campsite or on a job site |
Online calculators
An online calculator is the fastest way to size a generator before you buy one. Pick what you plan to run and it adds the numbers for you. Check that the tool asks for starting watts as well as running watts, because a generator that covers your running load can still trip when a fridge or a pump starts. The generator size calculator on this site uses typical figures for 30-plus household items, adds the biggest starting kick and gives you the two numbers to look for on a generator label.
The catch is the connection. During an outage, at a campsite or on a job site, you may not have one.
Wattage charts
A wattage chart lists typical running and starting watts per appliance. It works anywhere once printed, but you do the adding yourself, and it is easy to add every starting kick instead of only the biggest one, which oversizes the generator. Our house sizing guide has a chart for 15 household items, and the equipment worksheet is a printable place to record your own ratings.
A generator calculator app
An app keeps the calculator on your phone, so it works with no signal. Generator Wattage Calculator for iPhone lets you tap what you plan to plug in, checks it against your generator's running and starting watts, and tells you whether it can run them all. You can keep separate lists for home, RV, camp or the job site.
Which should you use?
Use the online calculator to choose a generator before you buy. Print a chart or the worksheet to keep with the generator. Put the app on your phone if you want to check loads when the power is out or you are away from a connection.
Questions
Is there an app that works out generator size offline?
Yes. Generator Wattage Calculator for iPhone works offline. Tap what you plan to run and it checks the total against your generator's running and starting watts.
Do online generator calculators count starting watts?
Some do and some only add running watts. Check that the tool asks for starting watts. The calculator on generatorsizes.com adds the single biggest starting kick to your running total.
Is a wattage chart enough to size a generator?
A chart gives you typical figures. Add the running watts of everything you run at the same time, then add only the biggest starting kick, and use the numbers on your own appliance labels where you can.